A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103, which in turn vibrate the oval window and round window membrane openings of the cochlea 104. The cochlea 104 is a narrow fluid-filled duct that is wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The scala tympani forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain. Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104.
In some cases, hearing impairment can be addressed by a cochlear implant that electrically stimulates auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along an implant electrode. FIG. 1 also shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external signal processing stage 111 which implements one of various known signal processing schemes. The processed audio signal is converted by the external signal processing stage 111 into a digital data format for transmission into an implant stimulator 108. Besides extracting the audio information, the implant stimulator 108 may perform additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through connected wires 109 to an implant electrode 110. Typically, the implant electrode 110 includes multiple stimulation electrodes on its surface that provide selective stimulation of the cochlea 104.
In cochlear implants, a relatively small number of stimulation electrodes are each associated with relatively broad frequency bands, with each stimulation electrode addressing a group of neurons with a stimulation pulse the charge of which is derived from the instantaneous amplitude of the signal envelope within the associated frequency band. In some coding strategies, stimulation pulses are applied at a constant rate across all stimulation electrodes, whereas in other coding strategies, stimulation pulses are applied at an electrode-specific rate.
One problem in cochlear implants is that of spatial channel interaction. Spatial channel interaction means that there is significant geometric overlapping of electrical fields at the location of the excited nervous tissue, if multiple different stimulation electrodes are activated at around the same time. Spatial channel interaction is primarily due to the conductive fluids and tissues surrounding the implant electrode 110.
One successful stimulation strategy is “Continuous-Interleaved-Sampling” (CIS) as introduced by Wilson at al., Better Speech Recognition with Cochlear Implants, Nature, vol. 352, 236-238, July 1991; incorporated herein by reference. CIS signal processing typically involves:                (1) splitting up of the audio frequency range into spectral bands by means of a filter bank;        (2) envelope detection of each filter output signal; and        (3) instantaneous nonlinear compression of the envelope signal (map law).Based on the tonotopic organization of the cochlea, each stimulation electrode in the scala tympani is associated with a band pass filter of the external filter bank and symmetrical biphasic current pulses are applied as stimulation. The amplitudes of the stimulation pulses are directly obtained from the compressed envelope signals (step (3) above). These signals are sequentially sampled and the stimulation pulses are applied in a strictly non-overlapping sequence. Thus, spatial channel interaction is minimized and a comparatively precise definition of electrical fields in the cochlea is achieved. For example, consider a 12-channel CIS-system with a maximum overall stimulation rate of 18 kpps; assuming that each channel is addressed once in a cycle, the stimulation rate per channel is 1.5 kpps. Such a stimulation rate per channel usually is sufficient for adequate temporal representation of the envelope signal. The maximum overall stimulation rate is limited by the minimum phase duration per pulse. The phase duration cannot be chosen arbitrarily short, because the shorter the pulses, the higher the current amplitudes have to be to elicit action potentials in neurons, and current amplitudes are limited for various practical reasons. For an overall stimulation rate of 18 kpps, the phase duration is 27 μs, which approaches the lower limit.        
A stimulation strategy related to CIS is the “N-of-M” strategy, wherein only the N electrode channels with maximum energy are selected out of the total number of M band pass signal channels during each stimulation cycle, as described by Wilson et al., Comparative Studies Of Speech Processing Strategies For Cochlear Implants, Laryngoscope 1998; 98:1069-1077; incorporated herein by reference. Typically, the number of band pass signal channels M is constant and equal to the overall number of usable channels. Thereby the instantaneous stimulation rate of a selected channel is increased by a factor of M/N. Interestingly, N of M strategies do not seem not to improve speech perception as compared to standard CIS, as described in Ziese et al., Speech Understanding With CIS And N-Of-M Strategy In The MED-EL COMBI 40+ System, ORL 2000; 62:321-329; incorporated herein by reference.
One disadvantage of N-of-M strategies (with constant M) is that neurons or groups of neurons may suffer “micro-shocks” if electrode channels are switched from “inactive” to “active”. For example, consider a situation where a train of supra-threshold stimulation pulses is applied at a particular stimulation electrode. The initial pulse will cause action potentials in most of the neighboring neurons, followed by a refractory period during which a more limited neural response can be elicited. Most of the neurons will continue to be in similar refractory states until enough time has passed to cause a sufficient distribution of refractory states. Thus, for at least an initial period of time, most of the neurons will respond in the same manner to each pulse due to their similar refractory state, as described by Wilson et al., Temporal Representation With Cochlear Implants, Am. J. Otology, Vol. 18, No. 6 (Suppl), S30-S34, 1997; incorporated herein by reference.
In conventional CIS, periods with no activity at particular stimulation electrodes do not occur since each electrode is stimulated in each cycle and minimum pulse amplitudes are usually close to or slightly above thresholds. So even when there is no spectral energy present in a particular frequency band, the associated electrode will be active keeping neurons in different refractory states. In addition, a number of neurons may be kept busy because of activity in neighboring channels. In this respect, spatial channel interaction can have an (unintentional) advantageous effect.
Another issue with N-of-M stimulation is the tendency for higher frequency signal channels to dominate over low frequency stimulation channels. This effect is especially unfortunate because of the fact that the lower frequency signal channels contain the fundamental frequency of the overall audio signal, which is the most dominant cue for speech understanding.